kvm tools: Fix 'kill -3' hangs

kvm tools: Fix 'kill -3' hangs

Ingo Molnar reported that 'kill -3' didn't work on his machine:

  * Ingo Molnar <mingo@elte.hu> wrote:

  > This is really cumbersome to debug - is there some good way to get to the RIP
  > that the guest is hanging in? If kvm would print that out to the host console
  > (even if it's just the raw RIP initially) on a kill -3 that would help
  > enormously.

  Looks like the code should be doing that already - but the ioctl(KVM_GET_SREGS)
  hangs:

    [pid   748] ioctl(6, KVM_GET_SREGS

Avi Kivity pointed out that it's not safe to call KVM_GET_SREGS (or other vcpu
related ioctls) from other threads:

  > is it not OK to call KVM_GET_SREGS from other threads than the one
  > that's doing KVM_RUN?

  From Documentation/kvm/api.txt:

   - vcpu ioctls: These query and set attributes that control the operation
     of a single virtual cpu.

     Only run vcpu ioctls from the same thread that was used to create the
     vcpu.

Fix that up by using pthread_kill() to force the threads that are doing KVM_RUN
to do the register dumps.

Reported: Ingo Molnar <mingo@elte.hu>
Cc: Asias He <asias.hejun@gmail.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>